From the second source, compiled by R’ Dovid Sears, a quote from the Maggid of Mezritch:

Contemplating the Word “Echad (One)”

“ ‘Hear O Israel, the Lord our God, the Lord is One’ (Deuteronomy 6:6). When during this part of the prayer service a person recites the word ‘One,’ he should contemplate that the Holy One, blessed be He, is all that truly exists in the universe, for ‘the entire world is filled with His Glory’ (Isaiah 6:3). One must realize that he is nothing, for the essence of a person is his soul, and the soul is but a ‘portion of God Above’ (Shefa Tal 1a). Therefore, nothing truly exists except the Holy One, blessed be He.”